Incompatibility of PCD2 /PCD3.F27x(0) and "Hydrometer M-Bus Receiver 868"

FAQ #101717

 

During our analysis it was found that the power consumption variation of the "Hydrometer M-Bus Receiver 868" while receiving data is higher than specified in the M-Bus specification. This leads to the fact that the communication between the F271x and the "Hydrometer M-Bus Receiver 868" receiver is not working.
Note that the manufacturer is explicitly specifying that this device is only to be used with "his" M-Bus master, see screenshot below.

Detailed information
Analyzed Radio Receiver: Hydrometer M-Bus Receiver 868
The device takes the power from the M-Bus communication lines; current consumption is around 20mA (instead of max. 1.5mA). So far, this is no problem as long as the device is counted as ~13x load. But when sending data on the M-Bus lines, the current consumption of this slave changes by more than 10mA, and this change is seen as incoming data on the M-Bus master side.

The M-Bus standard SN EN 13757-2 specifies:
Voltage variation of 1..15V shall not change the bus current by more than Nx3uA/V. N must be specified, values in the range from 1 to 4 are allowed. Max. change with SBC M-Bus master: 12V*4*3uA/V = 144uA.

Conclusion
The "Hydrometer M-Bus Receiver 868" does not comply with M-Bus standard. (and its manufacturer specifies that this Receiver has to be used only with their master devices). Therefore it can be (by chance) that it works with third party master devices. With the F27x modules the functionality is not guaranteed (guaranteed functionality would require a hardware redesign of the F27x to support the special device).

Remark
In Germany the "Hydrometer M-Bus Receiver 868" is distributed by Hydrometer while in Switzerland it is distributed by Techem and in France the same device is distributed by the company Sappel under the name "IZAR".